1. Field of the Invention
The present invention relates to a supplemental liquid developer for electrostatic photography, more specifically to an improvement of the life of a liquid developer for electrostatic photography by such a supplemental liquid developer.
2. Description of the Related Art
The following liquid developers for electrostatic photography are typical of those proposed to date:
One obtained by kneading a colorant such as carbon black, or an organic pigment or dye, a binder such as acrylic resin, a synthetic rubber, rosin or a rosin derivative, and a charge control agent such as an aliphatic or aromatic carboxylic acid, in a three-roll mill, a ball mill, etc., to disperse them in a nonpolar insulating liquid or solvent (see Japanese Examined Patent Publication (Kokoku) No. 51-1301).
One obtained by copolymerizing a monomer having a polymerizing active double bond, which is soluble in a nonpolar insulating carrier liquid but becomes insoluble after polymerization, for example, a vinyl ester of acetic acid, propionic acid, etc., a monomer containing an aliphatic group having more than 7 carbon atoms, which is soluble in or compatible with the above carrier liquid during polymerization, for example, 2-ethyl hexylmethacrylate, lauryl methacrylate, stearyl methacrylate, etc., and a monomer having a polar group and a plymerizing active double bond for charge control, for example, acrylic acid, methacrylic acid, maleic acid, anhydrous maleic acid, acrylamido, etc., in a nonpolar insulting carrier liquid to disperse a copolymer particle therein, followed by coloring it with an organic dye or pigment, etc. (see Japanese Unexamined Patent Publication (Kokai) No. 60-249160).
One obtained by first copolymerizing a monomer containing a double bond and more than 7 carbon atoms soluble or compatible, during polymerization, for example, 2-ethylhexyl methacrylate, lauryl methacrylate, stearyl acrylate, etc. and a monomer capable of esterification, for example, acrylic acid, glycidyl methacrylate, etc., in a nonpolar insulating carrier liquid; esterifying the resultant copolymer at a portion corresponding to the monomer for esterification, for example, glycidyl methacrylate, acrylic acid, to provide a pendant group having a double bond to the copolymer; polymerizing a monomer such as vinyl acetate, vinyl propionate, etc., to the copolymer using the double bond to make the copolymer insoluble in the nonpolar insulating carrier liquid and disperse a polymer particle therein; adding a metallic soap such as cobalt naphthanate or manganese octenate as a charge control agent, if necessary; followed by coloring with carbon black or an organic dye or pigment (see Japanese Examined Patent Publication (Kokoku) No. 56-10619, corresponding to U.S. Ser. No. 810841 filed on May 26, 1969 and U.S. Ser. No. 7253 filed on Jan. 30, 1970, and Japanese Unexamined Patent Publication (Kokai) No. 53-54029 ).
Schematically, these liquid developers for electrostatic photography are formed by dispersing, as main components, a colorant, a binder and a charge control agent in a nonpolar insulating carrier liquid, but the distinguishing of a colorant, a binder and a charge control agent is not strict, and in practice, one chemical ingredient may play all or a part of the roles of two components such as the colorant and the charge control agent.
In the development process, electrophoresis occurs in a liquid developer in accordance with an electrical charge of an electrostatic latent image formed on a surface of a photosensitive material for electrophotography or an electrostatic recording material, a toner in the developer adheres thereto, and an image is developed (visualized). After repeated developments, although a new developer is promptly supplemented, the image becoms unclear, i.e., tailing, decrease of the amount of adhesion, adhesion to a non-image portion, etc. occur, and an agglomeration of particles in the developer occurs to increase the particle size and cause precipitation thereof. Accordingly, a developer has only a certain life.